A structural role for hormone in the thyroid hormone receptor

The crystal structure of the rat alpha 1 thyroid hormone receptor ligand-binding domain bound with a thyroid hormone agonist reveals that ligand is completely buried within the domain as part of the hydrophobic core. In addition, the carboxy-terminal activation domain forms an amphipathic helix, with its hydrophobic face constituting part of the hormone binding cavity. These observations suggest a structural role for ligand, in establishing the active conformation of the receptor, that is likely to underlie hormonal regulation of gene expression for the nuclear receptors.

Nonalcoholic Fatty Liver Disease and Hypercholesterolemia: Roles of Thyroid Hormones, Metabolites, and Agonists

Background: Thyroid hormones (THs) exert a strong influence on mammalian lipid metabolism at the systemic and hepatic levels by virtue of their roles in regulating circulating lipoprotein, triglyceride (TAG), and cholesterol levels, as well as hepatic TAG storage and metabolism. These effects are mediated by intricate sensing and feedback systems that function at the physiological, metabolic, molecular, and transcriptional levels in the liver. Dysfunction in the pathways involved in lipid metabolism disrupts hepatic lipid homeostasis and contributes to the pathogenesis of metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD) and hypercholesterolemia. There has been strong interest in understanding and employing THs, TH metabolites, and TH mimetics as lipid-modifying drugs. Summary: THs regulate many processes involved in hepatic TAG and cholesterol metabolism to decrease serum cholesterol and intrahepatic lipid content. TH receptor β analogs designed to have less side effects than the natural hormone are currently being tested in phase II clinical studies for NAFLD and hypercholesterolemia. The TH metabolites, 3,5-diiodo-l-thyronine (T2) and T1AM (3-iodothyronamine), have different beneficial effects on lipid metabolism compared with triiodothyronine (T3), although their clinical application is still under investigation. Also, prodrugs and glucagon/T3 conjugates have been developed that direct TH to the liver. Conclusions: TH-based therapies show clinical promise for the treatment of NAFLD and hypercholesterolemia. Strategies for limiting side effects of TH are being developed and may enable TH metabolites and analogs to have specific effects in the liver for treatments of these conditions. These liver-specific effects and potential suppression of the hypothalamic/pituitary/thyroid axis raise the issue of monitoring liver-specific markers of TH action to assess clinical efficacy and dosing of these compounds.

Targeting protein-protein interaction by small molecules

Protein-protein interactions (PPIs) are critical regulatory events in physiology and pathology, and they represent an important target space for pharmacological intervention. However, targeting PPIs with small molecules is challenging owing to the large surface area involved in protein-protein binding and the lack of obvious small-molecule-binding pockets at many protein-protein interfaces. Nonetheless, successful examples of small-molecule modulators of PPIs have been growing in recent years. This article reviews some of the recent advances in the discovery of small-molecule regulators of PPIs that involve key oncogenic proteins. Our discussion focuses on the three key modes of action for these small-molecule modulators: orthosteric inhibition, allosteric regulation, and interfacial binding/stabilization. Understanding the opportunities and challenges of these diverse mechanisms will help guide future efforts in developing small-molecule modulators against PPIs.

An in vivo multiwell-based fluorescent screen for monitoring vertebrate thyroid hormone disruption

There is a pressing need for high throughput methods to assess potential effects of endocrine disrupting chemicals (EDCs). released into the environment. Currently our ability to identify effects in vitro exceeds that for in vivo monitoring. However, only in vivo analysis provides the full spectrum of physiological impacts exerted by a given chemical. With the aim of finding a physiological system compatible with automatic plate reading we tested the capacity of early embryonic stage Xenopus laevis tadpoles to monitor thyroid hormone (TH) disruption. Fluorescent transgenic X. laevis embryos bearing a TH/bZIP-eGFP construct, placed in 96 well plates, were used for a physiological-based screen for potential TH signaling disruptors. Using stage NF-45 embryos (time of thyroid gland formation) allowed rapid detection of chemical interference with both peripheral TR signaling and production of endogenous TH. Nanomolar concentrations of TH receptor agonists could be detected within 72 h. Moreover, when testing against a 5nM T3 challenge, the effects of inhibitors of TH production were revealed, including inhibitors of TH synthesis, (methimazole: 1 mM or sodium perchlorate: 3.56 microM), as well as antagonists acting at the receptor level (NH3: 2 microM) and a deiodinase inhibitor (iopanoic acid: 10 microM). Finally, we show that the thyroid disrupting activities of BPA (10 microM) and TBBPA (1 microM) can also be detected in this rapid screening protocol. Finally, this noninvasive technology using an automatic reading system shows low variability (around 5%) and permits detection of subtle changes in signaling by EDCs that either inhibit or activate TH signaling in vivo.

3,5-Diiodo-L-thyronine (T2) has selective thyromimetic effects in vivo and in vitro

Recent data have suggested that the iodothyronine, 3,5-diiodo-l-thyronine (T2), has selective thyromimetic activity. In vivo, T2 has been shown to suppress TSH levels at doses that do not produce significant peripheral manifestations of thyroid hormone activity. Furthermore, T2 has been shown to produce smaller increments in peripheral indices of thyroid status than does T3, when doses resulting in equivalent suppression of circulating TSH are compared. We have assessed the selective thyromimetic activity of T2 in vivo and in vitro, and performed in vitro studies to assess the potential molecular basis for these selective properties. T2 was 100-fold less potent than T3 in stimulating GH mRNA levels in GH3 cells. In contrast, the iodothyronines were almost equivalent in their ability to downregulate TRbeta2 mRNA levels in this cell line. Both 3,3'-diiodo-L-thyronine and thyronine exhibited no significant thyromimetic effects on either process. In vivo, doses of T2 and T3 that were equivalent in their induction of hepatic malic enzyme (ME) mRNA did not produce equivalent suppression of circulating TSH, with T2 being only 27% as effective as T3. T2 was up to 500-fold less potent than T3 in displacing [125I]-T3 from in vitro translated specific nuclear receptors (TRs) and GH3 cell nuclear extracts. Electrophoretic mobility shift assays, assessing the ability of T2 to produce dissociation of TRbeta1 homodimers from inverted palindrome T3 response elements, indicated that T2 was also 1000-fold less potent than T3 in this respect. These data confirm that T2 has significant thyromimetic activity, and that this activity is selective both in vivo and in vitro. However, there are no data to support a selective central effect, T2 being relatively more potent in stimulating hepatic ME mRNA than in suppression of TSH in vivo. The basis for this differential thyromimetic activity is not selective affinity of the different TR isoforms for T2, or divergent properties of T2 in competitive binding and functional assays in vitro.

Myelin repair stimulated by CNS-selective thyroid hormone action

Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology, and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair.

Analysis of thyroid hormone receptor betaA mRNA expression in Xenopus laevis tadpoles as a means to detect agonism and antagonism of thyroid hormone action

Amphibian metamorphosis represents a unique biological model to study thyroid hormone (TH) action in vivo. In this study, we examined the utility of thyroid hormone receptors alpha (TRalpha) and betaA (TRbetaA) mRNA expression patterns in Xenopus laevis tadpoles as molecular markers indicating modulation of TH action. During spontaneous metamorphosis, only moderate changes were evident for TRalpha gene expression whereas a marked up-regulation of TRbetaA mRNA occurred in hind limbs (prometamorphosis), head (late prometamorphosis), and tail tissue (metamorphic climax). Treatment of premetamorphic tadpoles with 1 nM 3,5,3'-triiodothyronine (T3) caused a rapid induction of TRbetaA mRNA in head and tail tissue within 6 to 12 h which was maintained for at least 72 h after initiation of T3 treatment. Developmental stage had a strong influence on the responsiveness of tadpole tissues to induce TRbetaA mRNA during 24 h treatment with thyroxine (0, 1, 5, 10 nM T4) or T3 (0, 1, 5, 10 nM). Premetamorphic tadpoles were highly sensitive in their response to T4 and T3 treatments, whereas sensitivity to TH was decreased in early prometamorphic tadpoles and strongly diminished in late prometamorphic tadpoles. To examine the utility of TRbetaA gene expression analysis for detection of agonistic and antagonistic effects on T3 action, mRNA expression was assessed in premetamorphic tadpoles after 48 h of treatment with the synthetic agonist GC-1 (0, 10, 50, 250 nM), the synthetic antagonist NH-3 (0, 40, 200, 1000 nM), and binary combinations of NH-3 (0, 40, 200, 1000 nM) and T3 (1 nM). All tested concentrations of GC-1 as well as the highest concentration of NH-3 caused an up-regulation of TRbetaA expression. Co-treatment with NH-3 and T3 revealed strong antagonistic effects by NH-3 on T3-induced TRbetaA mRNA up-regulation. Results of this study suggest that TRbetaA mRNA expression analysis could serve as a sensitive molecular testing approach to study effects of environmental compounds on the thyroid system in X. laevis tadpoles.

Pharmacologic activation of PKM2 slows lung tumor xenograft growth

Inactivation of the M2 form of pyruvate kinase (PKM2) in cancer cells is associated with increased tumorigenicity. To test the hypothesis that tumor growth may be inhibited through the PKM2 pathway, we generated a series of small-molecule PKM2 activators. The compounds exhibited low nanomolar activity in both biochemical and cell-based PKM2 activity assays. These compounds did not affect the growth of cancer cell lines under normal conditions in vitro, but strongly inhibited the proliferation of multiple lung cancer cell lines when serine was absent from the cell culture media. In addition, PKM2 activators inhibited the growth of an aggressive lung adenocarcinoma xenograft. These findings show that PKM2 activation by small molecules influences the growth of cancer cells in vitro and in vivo, and suggest that such compounds may augment cancer therapies.

Discovery of 3-(trifluoromethyl)-1H-pyrazole-5-carboxamide activators of the M2 isoform of pyruvate kinase (PKM2)

Activators of the pyruvate kinase M2 (PKM2) are currently attracting significant interest as potential anticancer therapies. They may achieve a novel antiproliferation response in cancer cells through modulation of the classic 'Warburg effect' characteristic of aberrant metabolism. In this Letter, we describe the optimization of a weakly active screening hit to a structurally novel series of small molecule 3-(trifluoromethyl)-1H-pyrazole-5-carboxamides as potent PKM2 activators.

Discovery of 2-((1H-benzo[d]imidazol-1-yl)methyl)-4H-pyrido[1,2-a]pyrimidin-4-ones as novel PKM2 activators

The M2 isoform of pyruvate kinase is an emerging target for antitumor therapy. In this letter, we describe the discovery of 2-((1H-benzo[d]imidazol-1-yl)methyl)-4H-pyrido[1,2-a]pyrimidin-4-ones as potent and selective PKM2 activators which were found to have a novel binding mode. The original lead identified from high throughput screening was optimized into an efficient series via computer-aided structure-based drug design. Both a representative compound from this series and an activator described in the literature were used as molecular tools to probe the biological effects of PKM2 activation on cancer cells. Our results suggested that PKM2 activation alone is not sufficient to alter cancer cell metabolism.

Activators of PKM2 in cancer metabolism

Pyruvate kinase converts phosphoenolpyruvate to pyruvate, catalyzing the rate-limiting step of glycolysis. The M1 isoenzyme of pyruvate kinase (PKM1) is found in adult tissues; whereas, PKM2 is a splicesome variant found in embryonic and cancer cells. PKM2 expression in malignant cells is a result of the tumor microenvironment and is responsible for maintaining a glycolytic phenotype. PKM2 has other nonmetabolic functions in malignant cells, including transcriptional coactivation and protein kinase activity. PKM2 activators have antitumor properties by inducing tetramerization of two PKM2 dimers causing PKM2 to function like PKM1. Restoring PKM2 to PKM1-like levels of activity causes reversal of the Warburg effect in cancer cells. PKM2 activators have therapeutic potential in the treatment of cancer and other metabolic diseases.

1-(sulfonyl)-5-(arylsulfonyl)indoline as activators of the tumor cell specific M2 isoform of pyruvate kinase

Cancer cells preferentially use glycolysis rather than oxidative phosphorylation for their rapid growth. They consume large amount of glucose to produce lactate even when oxygen is abundant, a phenomenon known as the Warburg effect. This metabolic change originates from a shift in the expression of alternative spliced isoforms of the glycolytic enzyme pyruvate kinase (PK), from PKM1 to PKM2. While PKM1 is constitutively active, PKM2 is switched from an inactive dimer form to an active tetramer form by small molecule activators. The prevalence of PKM2 in cancer cells relative to the prevalence of PKM1 in many normal cells, suggests a therapeutic strategy whereby activation of PKM2 may counter the abnormal cellular metabolism in cancer cells, and consequently decreased cellular proliferation. Herein we describe the discovery and optimization of a series of PKM2 activators derived from the 2-((2,3-dihydrobenzo[b][1,4] dioxin-6-yl)thio)-1-(2-methyl-1-(methylsulfonyl)indolin-5-yl) ethanone scaffold. The synthesis, SAR analysis, enzyme active site docking, enzymatic reaction kinetics, selectivity and pharmaceutical properties are discussed.

Bisphenol A acts differently from and independently of thyroid hormone in suppressing thyrotropin release from the bullfrog pituitary

The objective of this investigation was to ascertain whether bisphenol A (BPA), which has a structural resemblance to thyroid hormone (TH), acts as a TH agonist or antagonist in terms of affecting the release of thyrotropin (TSH). To this end, we exposed adult bullfrog (Rana catesbeiana) pituitary cells to BPA and/or TH in the presence or absence of corticotropin-releasing factor (CRF), which is known to have a potent TSH-releasing activity in amphibians. BPA (10(-9)-10(-4)M) did not affect the basal release of TSH. However, it suppressed CRF-inducible TSH release at 10(-4)M, but not at 10(-5)M. Triiodothyronine (T(3)) at 10(-7)M and l-thyroxine (T(4)) at 10(-6)M also suppressed the CRF-inducible release of TSH. The combination of T(3) (10(-7)M) or T(4) (10(-6)M) with BPA (10(-4)M) had an additive effect in suppressing TSH release. A comparison of the suppressive effects of BPA and T(3) on the release of TSH following the addition of actinomycin D or cycloheximide to the culture medium revealed that both of the latter compounds blocked T(3)-inducible but not BPA-inducible suppression of TSH release. The results indicate that the mechanism of action of BPA is different from that of T(3) in that T(3) action involves RNA and protein synthesis, whereas BPA action does not involve either of these processes. Furthermore, BPA was found to suppress the thyrotropin-releasing hormone-inducible release of both prolactin (PRL) and TSH. Our results suggest that BPA acts not only as a blocker of TSH secretagogues but also as a blocker of a PRL secretagogue at the pituitary level. Estradiol affected neither the release of TSH nor the release of PRL in the presence or absence of their secretagogues, suggesting that the suppression of the release of TSH and PRL caused by BPA may not be derived from its estrogenic activity.

Polychlorinated biphenyls (Aroclor 1254) do not uniformly produce agonist actions on thyroid hormone responses in the developing rat brain

Thyroid hormone (TH) is essential for normal brain development, and polychlorinated biphenyls (PCBs) are known to interfere with TH action in the developing brain. Thus, it is possible that the observed neurotoxic effects of PCB exposure in experimental animals and humans are mediated in part by their ability to interfere with TH signaling. PCBs may interfere with TH signaling by reducing circulating levels of TH, acting as TH receptor analogs, or both. If PCBs act primarily by reducing serum TH levels, then their effects should mimic those of low TH. In contrast, if PCBs act primarily as TH agonists in the developing brain, then they should mimic the effect of T(4) in hypothyroid animals. We used a two-factor design to test these predictions. Both hypothyroidism (Htx) and/or PCB treatment reduced serum free and total T(4) on postnatal d 15. However, only Htx increased pituitary TSHbeta expression. RC3/neurogranin expression was decreased by Htx and increased by PCB treatment. In contrast, Purkinje cell protein-2 expression was reduced in hypothyroid animals and restored by PCB treatment. Finally, PCB treatment partially ameliorated the effect of Htx on the thickness of the external granule layer of the cerebellum. These studies demonstrate clearly that PCB exposure does not mimic the effect of low TH on several important TH-sensitive measures in the developing brain. However, neither did PCBs mimic T(4) in hypothyroid animals on all end points measured. Thus, PCBs exert a complex action on TH signaling in the developing brain.

Evaluation of gene expression endpoints in the context of a Xenopus laevis metamorphosis-based bioassay to detect thyroid hormone disruptors

Thyroid hormones (TH) are important in growth, development and the maintenance of proper cellular metabolism in vertebrates. Amphibian metamorphosis is completely dependent on TH and forms the basis of a screen for thyroid axis disrupting chemicals that currently relies on external morphological endpoints and changes in thyroid gland histology. The requirement for TH-dependent gene expression makes it possible to augment this screen through the addition of molecular endpoints. In order to do this, gene selection, choice of sampling time, tissue sensitivity, and their relationship to morphological change must all be considered. We exposed stage 54 Xenopus laevis tadpoles to a concentration series of the THs, thyroxine (T4) and 3,5,3'-triiodothyronine (T3), and three known TH antagonists, methimazole, propylthiouracil (PTU), and perchlorate. The agonists significantly accelerated metamorphosis as defined by developmental stage attained after 14 days. In contrast, the TH antagonists significantly delayed metamorphosis at 14 days and caused an increase in thyroid gland size at day 8. We assessed the changes in steady-state mRNA levels of thyroid hormone receptor alpha- and beta-isoforms and the basic transcription element binding (BTEB) protein by quantitative real-time polymerase chain reaction. Three tissues (brain, tail and hindlimb) were analyzed at 24, 48 and 96 h and we found that TH receptor, TRbeta, and BTEB were the most sensitive gene transcripts for the TH agonists, whereas only TRalpha displayed significant changes upon antagonist exposure. We detected differences in tissue-specific responses between the two agonists. We matched the concentrations of T3 and T4 that elicited similar biological responses at 14 days and compared the induction of gene expression. At 96 h, the TRbeta and BTEB expression response to T3 and T4 was similar in the tail. In contrast, T3 elicited no concentration-dependent changes in TRbeta and BTEB expression in the brain, whereas T4 elevated their expression. The tail showed the highest correlation between TH concentration and morphological outcome whereas the brain was the most sensitive to antagonist treatment. Only methimazole and perchlorate showed significant changes in TRalpha gene expression in the brain whereas PTU did not suggesting differences in cellular mechanisms of action. The greatest effect on gene expression occurred within 48 h with many of the hormone-dependent changes disappearing by 96 h. This study accentuates the need to examine multiple tissues and provides critical information required for optimization of exposure regimens and endpoint assessments that focus on the detection of disruption in TH-regulatory systems.

Identification of gene expression indicators for thyroid axis disruption in a Xenopus laevis metamorphosis screening assay. Part 1. Effects on the brain

Thyroid hormones (TH), thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)), play crucial roles in regulation of growth, development and metabolism in vertebrates and their actions are targets for endocrine disruptive agents. Perturbations in TH action can contribute to the development of disease states and the US Environmental Protection Agency is developing a high throughput screen using TH-dependent amphibian metamorphosis as an assay platform. Currently this methodology relies on external morphological endpoints and changes in central thyroid axis parameters. However, exposure-related changes in gene expression in TH-sensitive tissue types that occur over shorter time frames have the potential to augment this screen. This study aims to characterize and identify molecular markers in the tadpole brain. Using a combination of cDNA array analysis and real time quantitative polymerase chain reaction (QPCR), we examine the brain of tadpoles following 96 h of continuous exposure to T(3), T(4), methimazole, propylthiouracil, or perchlorate. This tissue was more sensitive to T(4) rather than T(3), even when differences in biological activity were taken into account. This implies that a simple conversion of T(4) to T(3) cannot fully account for T(4) effects on the brain and suggests distinctive mechanisms of action for the two THs. While the brain shows gene expression alterations for methimazole and propylthiouracil, the environmental contaminant, perchlorate, had the greatest effect on the levels of mRNAs encoding proteins important in neural development and function. Our data identify gene expression profiles that can serve as exposure indicators of these chemicals.

Role of PKM2-Mediated Immunometabolic Reprogramming on Development of Cytokine Storm

The cytokine storm is a marker of severity of various diseases and increased mortality. The altered metabolic profile and energy generation of immune cells affects their activation, exacerbating the cytokine storm. Currently, the emerging field of immunometabolism has highlighted the importance of specific metabolic pathways in immune regulation. The glycolytic enzyme pyruvate kinase M2 (PKM2) is a key regulator of immunometabolism and bridges metabolic and inflammatory dysfunction. This enzyme changes its conformation thus walks in different fields including metabolism and inflammation and associates with various transcription factors. This review summarizes the vital role of PKM2 in mediating immunometabolic reprogramming and its role in inducing cytokine storm, with a focus on providing references for further understanding of its pathological functions and for proposing new targets for the treatment of related diseases.

Thyroid hormone promotes differentiation of colon cancer stem cells

Tumor formation and maintenance depend on a small fraction of cancer stem cells (CSCs) that can self-renew and generate a wide variety of differentiated cells. CSCs are resistant to chemotherapy and radiation, and can represent a reservoir of cancer cells that often cause relapse after treatment. Evidence suggests that CSCs also give rise to metastases. Thyroid hormone (TH) controls a variety of biological processes including the development and functioning of most adult tissues. Recent years has seen the emergence of an intimate link between TH and multiple steps of tumorigenesis. Thyroid hormone controls the balance between the proliferation and differentiation of CSCs, and may thus be a druggable anti-cancer agent. Here, we review current understanding of the effects of TH on colorectal CSCs, including the cross regulatory loops between TH and regulators of CSC stemness. Targeting TH in the tumor microenvironment may improve treatment strategies.

Synthesis of novel 7-azaindole derivatives containing pyridin-3-ylmethyl dithiocarbamate moiety as potent PKM2 activators and PKM2 nucleus translocation inhibitors

Multiple lines of evidence have indicated that pyruvate kinase M2 (PKM2) is upregulated in most cancer cells and it is increasingly recognized as a potential therapeutic target in oncology. In a continuation of our discovery of lead compound 5 and SAR study, the 7-azaindole moiety in compound 5 was systematically optimized. The results showed that compound 6f, which has a difluoroethyl substitution on the 7-azaindole ring, exhibited high PKM2 activation potency and anti-proliferation activities on A375 cell lines. In a xenograft mouse model, oral administration of compound 6f led to significant tumor regression without obvious toxicity. Further mechanistic studies revealed that 6f could influence the translocation of PKM2 into nucleus, as well as induction of apoptosis and autophagy of A375 cells. More importantly, compound 6f significantly inhibited migration of A375 cells in a concentration-dependent manner. Collectively, 6f may serve as a lead compound in the development of potent PKM2 activators for cancer therapy.

Targeting pyruvate kinase muscle isoform 2 (PKM2) in cancer: What do we know so far?

Cancer is a leading cause of death globally. Cancer cell transformation is the result of intricate crosstalk between intracellular components and proteins. A characteristic feature of cancer cells is the ability to reprogram their metabolic pathways to ensure their infinite proliferative potential. Pyruvate kinase muscle isoform 2 (PKM2) is a glycolytic enzyme that plays crucial roles in cancer, apart from carrying out its metabolic roles. PKM2 is involved in all the major events associated with cancer growth. Modulation of PKM2 activity (dimer inhibition or tetramer activation) has been successful in controlling cancer. However, recent studies provide contrary evidences regarding the oncogenic functions of PKM2. Moreover, several studies have highlighted the cancerous roles of PKM1 isoform in certain contexts. The present review aims at providing the current updates regarding PKM2 targeting in cancer. Further, the review discusses the contradictory results that suggest that both the isoforms of PKM can lead to cancer growth. In conclusion, the review emphasizes revisiting the approaches to target cancer metabolism through PKM to find novel and effective targets for anticancer therapy.

Effects of nonylphenol on key hormonal balances and histopathology of the endangered Caspian brown trout (Salmo trutta caspius)

Endocrine disruptor chemicals (EDCs) potentially pose a hazard to endangered species. Evaluation of the sensitivity of these species to EDCs could be helpful for protecting their populations. So, the present study investigated the adverse effects of nonylphenol, an EDC, on the endocrine hormones and histopathology of male and female juvenile Caspian brown trout (Salmo trutta caspius) following 21 days of exposure to nominal concentrations of 1, 10 and 100 μg/l. The results showed that the HSI and plasma total calcium of male and female fishes exposed to 100 μg/l nonylphenol were significantly increased compared with the control groups (P<0.001). The male plasma T3 level was significantly decreased in 10 (P<0.01) and 100 (P<0.001) μg/l nonylphenol. The female T3 level increased in 1 μg/l nonylphenol concentration (P<0.05). The plasma T4 of males showed significant elevation in fishes exposed to 100 μg/l nonylphenol (P<0.05), but no change for females in any of treatment groups relative to controls (P>0.05). No significant effect of nonylphenol exposure was observed on male plasma TSH levels (P>0.05), whereas, in females, nonylphenol at all concentrations significantly reduced TSH levels. A bell-shaped response was observed in male and female plasma GH levels. Moreover, various histopathological lesions were observed in gill and intestine tissues of fishes exposed to different nonylphenol concentrations. These results demonstrate the high sensitivity of this endangered species to even environmentally relevant concentrations of nonylphenol. Furthermore, Caspian brown trout could be used as bioindicators reflecting the toxicity of nonylphenol.

Identification of gene expression indicators for thyroid axis disruption in a Xenopus laevis metamorphosis screening assay. Part 2. Effects on the tail and hindlimb

Thyroid hormones (TH), thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)), play crucial roles in regulation of growth, development and metabolism in vertebrates and are targets for endocrine disruptive agents. Perturbations in TH action can contribute to the development of disease states and the US Environmental Protection Agency is developing a high throughput screen using TH-dependent metamorphosis of the Xenopus laevis tadpole as an assay platform. Currently this methodology relies on external morphological endpoints and changes in central thyroid axis parameters. However, exposure-related changes in gene expression in TH-sensitive tissue types that occur over shorter time frames have the potential to augment this screen. Using a combination of cDNA array and real time quantitative polymerase chain reaction (QPCR) analyses, this study identifies molecular markers in tissues peripheral to the central thyroid axis. We examine the hindlimb and tail of tadpoles up to 96 h of continuous exposure to T(3), T(4), methimazole, propylthiouracil, or perchlorate. Several novel biomarker candidates are indicated that include transcripts encoding importin, RNA helicase II/Gu, and defender against death protein, DAD1. In combination with previously-identified biomarker candidates, these transcripts will greatly augment the predictive and diagnostic power of the Xenopus metamorphosis assay for perturbation of TH action.

Bioanalytical and instrumental analysis of thyroid hormone disrupting compounds in water sources along the Yangtze River

Thyroid hormone (TH) agonist and antagonist activities of water sources along the Yangtze River in China were surveyed by a green monkey kidney fibroblast (CV-1) cell-based TH reporter gene assay. Instrumental analysis was conducted to identify the responsible thyroid-active compounds. Instrumentally derived l-3,5,3'-triiodothyronine (T(3)) equivalents (T(3)-EQs) and thyroid receptor (TR) antagonist activity equivalents referring to dibutyl phthalate (DBP-EQs) were calculated from the concentrations of individual congeners. The reporter gene assay demonstrated that three out of eleven water sources contained TR agonist activity equivalents (TR-EQs), ranging from 286 to 293 ng T(3)/L. Anti-thyroid hormone activities were found in all water sources with the TR antagonist activity equivalents referring to DBP (Ant-TR-EQs), ranging from 51.5 to 555.3 μg/L. Comparisons of the equivalents from instrumental and biological assays suggested that high concentrations of DBP and di-2-ethylhexyl phthalate (DEHP) were responsible for the observed TR antagonist activities at some locations along the Yangtze River.

A yeast bioassay for direct measurement of thyroid hormone disrupting effects in water without sample extraction, concentration, or sterilization

The present study introduces an improved yeast bioassay for rapid yet sensitive evaluation of thyroid hormone disruption at the level of thyroid receptor (TR) in environmental water samples. This assay does not require water sample preparation and thus requires very little hands-on time. Based on different β-galactosidase substrates, two modified bioassays, a colorimetric bioassay and a chemiluminescent bioassay, were developed. The compounds tested included the known thyroid hormone 3,3',5-triiodo-l-thyronine (T3), the specific TR antagonist amiodarone hydrochloride (AH) and phthalate esters (PAEs), which potentially disrupt thyroid hormone signaling. The EC50 values for T3 were similar to those previously obtained using a 96-well plate bioassay. TR antagonism by AH was studied in the presence of 2.5 × 10(-7)M T3, and the concentration producing 20% of the maximum effect (RIC20) for AH was 3.1 × 10(-7)M and 7.8 × 10(-9)M for the colorimetric bioassay and chemiluminescent bioassay, respectively. None of the tested PAEs induced β-galactosidase expression, but diethylhexyl phthalate, benzyl butyl phthalate and dibutyl phthalate demonstrated TR antagonism. Furthermore, water samples collected from Guanting reservoir in Beijing were evaluated. Although TR agonism was not observed, antagonism was detected in all water samples and is expressed as AH equivalents. The toxicology equivalent quantity values obtained by the chemiluminescent bioassay ranged from 21.2 ± 1.6 to 313.9 ± 28.8 μg L(-1) AH, and similar values were obtained for the colorimetric bioassay. The present study shows that the modified yeast bioassay can be used as a valuable tool for quantification of thyroid hormone disrupting effects in environmental water samples.

Selective agonists and antagonists to thyroid hormone action

Selective thyromimetics have been designed and shown to exhibit some of the beneficial effects of thyroid hormones, such as lowering of cholesterol and weight reduction, without the adverse thyroid hormone action on muscle, bone, and heart rate. Progress has also been made in attempting to treat hyperthyroidism by synthesizing antagonists that block thyroid hormone action, at the level of the thyroid hormone receptor or of the thyrotropin receptor. Clinical trials are still awaited, however, to verify whether these potentially promising agents will indeed prove to be of clinical therapeutic value.